The present invention relates to gamma cameras and more particularly to an apparatus and method used with a line transmission source to modify the effective length of the line source thereby improving camera image quality and decreasing imaging time.
Single photon emission computed tomography (SPECT) examinations are carried out by injecting a dilution marker comprising a compound labeled with a radiopharmaceutical into the body of a patient to be examined. A radiopharmaceutical is a substance that emits photons at one or more energy levels. By choosing a compound that will accumulate in an organ to be imaged, compound concentration, and hence radiopharmaceutical concentration, can be substantially limited to an, organ of interest. A radiopharmaceutical that emits photons or gamma emissions which are approximately at a, single known energy level is chosen.
While moving through a patient's blood stream the marker, including the radiopharmaceutical, becomes concentrated in the organ to be imaged. By measuring the number of photons emitted from the organ which are at approximately the known energy range, organ characteristics, including irregularities, can be identified.
To measure the number of emitted photon a planar gamma camera is used. A gamma camera consists of a stand that supports a collimator, a scintillation crystal and a plurality of photomultiplier tubes (PMTs). The collimator typically includes a rectangular lead block having a width dimension and a length dimension which together define a camera field of view (FOV). The collimator block forms tiny holes which pass therethrough defining preferred photon paths. The preferred paths are usually unidirectional and perpendicular to the length of the collimator. The collimator blocks emissions toward the crystal along non-preferred paths.
After a marker has become concentrated within an organ to be imaged, a portion of a patient's body including the organ is positioned within the camera FOV. For the purposes of this explanation, while the organ and portion of a patient's body to be imaged can be any organ and surrounding biological tissue, it will be assumed that the organ to be imaged is an appendix, the portion of the body placed within the FOV includes a human torso and the portion of the torso including the appendix is a swath twenty centimeters long. The twenty centimeter swath will be referred to hereinafter as the torso segment.
The scintillation crystal is positioned adjacent the collimator on a side opposite the patient. The crystal detects photons that pass through the collimator on a front surface and emits light from a back surface each time a photon is detected. The amount of light emitted depends on the detected photon's energy level.
The PMTs are positioned adjacent the crystal and on a side of the crystal opposite the collimator. Light emitted by the crystal is detected by the PMTs which in turn generate analog intensity signals. A processor receives the intensity signals and digitally stores corresponding information as an M by N array of elements called pixels. Together the array of pixel information is used by the processor to form an emission image corresponding to the specific camera position.
Most gamma camera systems generate a plurality of emission images, each taken by positioning the detector parallel to, and at an angle about, a rotation axis which passes through the organ to be imaged. The angle is incremented between views so that the plurality of images can be used to construct pictures of transaxial slices of the torso section using algorithms and iterative methods that are well known to those skilled in the tomographic arts.
Because different materials are characterized by different attenuation coefficients, photons are attenuated to different degrees as they pass through different portions of the torso segment. For example, an inch of bone will typically attenuate a greater percentage of photons than an inch of tissue. Similarly, air filled space in a lung cavity or the like will attenuate less photons than a comparable space filled with tissue or bone. In addition, photons passing through four inches of tissue will be attenuated to a greater degree than photons passing through one inch of tissue. Thus, because the appendix is located on one side of a body, photon density on the appendix side of the body will typically be greater than density on the other side. Non-uniform attenuation about the appendix causes emission imaging errors such as image artifacts which can obscure images and reduce diagnostic effectiveness.
Attenuation caused by different biological materials within the torso segment can be compensated for by generating a body attenuation map and using the attenuation map to correct emission images. An attenuation map is a map which clearly indicates attenuation characteristics of different portions of the torso segment. For example, a map for the torso segment would indicate little attenuation in an air filled cavity, relatively greater attenuation in a muscle fiber and still greater attenuation in a bone section.
Torso segment attenuation can be directly measured by using transmission computed tomography techniques wherein the torso segment is positioned within a three dimensional imaging area and a source projects an even flow of photons through the imaging area toward a planar gamma camera like the emission camera described above. The distance between the source and camera defines one imaging area dimension. The other two imaging area dimensions are defined by the camera's FOV.
When a torso segment is positioned within a FOV, typically the segment will only block a portion of the FOV. In this case, while some of the transmission photons have flight paths which intersect the torso segment, other photons have paths which do not intersect the torso segment. This is particularly true when a torso segment is imaged from the side as most torso segments have a relatively narrow girth when compared to torso width. Hereinafter, photons having paths which intersect the torso segment will be referred to as intersecting photons. Photons having paths which do not intersect the torso segment will be referred to as non-intersecting photons.
Radiation received by the camera on the opposite side of the patient includes intersecting photons which are not attenuated by the torso segment and all non-intersecting photons. As with emission imaging, in transmission imaging, the scintillation crystal generates light each time a photon is detected and PMTs detect the light and generate intensity signals corresponding to each detected photon. A camera processor receives the intensity signals and performs a series of calculations corresponding to each detected photon to determine photon impact position. The position information is then used to generate a transmission image.
The source and detector are rotated about the torso segment to generate transmission images corresponding to a multiplicity of angles. The transmission images are reconstructed into the attenuation map using conventional tomography algorithms.
By collecting data corresponding to the intensity of the photon emissions and the intensity of the photon transmissions through the torso segment at the same gantry angles, a processor may use the non-uniform attenuation map to correct emission images collected during emission studies.
Because a planar source often causes excessive photon scatter, nearly all transmission sources are scanning line transmission sources. As the name implies, a scanning line transmission source is a source which generates a line beam of photons for imaging. Because the line source only generates a single line of photons, the line source can only be used to generate imaging data corresponding to a single slice of the torso segment at a time. To generate imaging data corresponding to the entire torso segment, the source is typically mounted for movement along a track. The track is usually parallel to a first camera dimension (e.g. width) and the source is mounted perpendicular to the track and therefore parallel to the second camera dimension (e.g. length). After the source is used to image one segment slice, the source is moved along the track to an adjacent location and is used to image an adjacent and parallel slice. In the present example, this process of moving and imaging continues across the entire twenty centimeter torso segment generating data corresponding to all segment slices within the segment length. In the alternative, the line source is scanned across the imaging area to generate imaging data for the entire torso segment, hence the term "scanning" line source.
The camera used for transmission imaging can be either a dedicated transmission camera separate from the emission camera or it can be the emission camera. In either case, the transmission photons are at a different energy level than the emission photons, and transmission imaging and emission imaging can be performed at the same time.
Perhaps the most important criterion for judging the usefulness of any transmission imaging system is the quality of the images generated by the system. In order to generate a useable image, at least a threshold number T.sub.P of intersecting photons which pass through the torso segment have to be detected by the scintillation crystal. For example, it may be necessary to detect at least 100,000 intersecting photons per centimeter slice of the torso segment to construct a useful transmission image. Where the torso segment is 20 centimeters long, the threshold number of intersecting photons T.sub.P would be 2 million.
For the purpose of this explanation, the term source activity will be used to refer to the number of photons generated by a source per second per centimeter of line beam length. In addition, the time required to scan a line transmitter across a FOV to generate an image at one imaging angle will be referred to as an imaging period. Moreover, the time required to generate images at all angles required to generate a tomographic image will be referred to as an imaging session.
Given any source activity, the threshold number T.sub.P of detected intersecting photons required to generate an image can be achieved by simply adjusting the duration of the imaging period. In the example above, where 100,000 photons have to be detected per centimeter slice of the torso segment and the slice is 20 centimeters long, assuming a first source activity which results in an average of 50,000 intersecting photon detected per second, the transmitter can be moved across the FOV at a rate of 0.5 cm/sec so that a total imaging period T.sub.I is 40 seconds. Period T.sub.I can be expresses as: ##EQU1## where P.sub.AI is the number of detected intersecting photons per second. At a second and reduced source activity resulting in 10,000 intersecting photon detected per second, the source can be moved across the FOV at a rate of 0.10 cm/sec so that, according to Equation 1 the total imaging period T.sub.I is extended to 200 seconds. In either case, the threshold number of detected intersecting photons is achieved and a useable image can be generated.
Unfortunately, in addition to image quality, another important system criterion is patient throughput. Imaging systems are relatively expensive diagnostic tools and therefore the cost of such systems is usually only justifiable where a large number of patients are examined each day. High throughput is also important for other reasons. For example, speedy imaging sessions advantageously minimize patient discomfort. Many patients are uncomfortable lying still during long imaging periods. While an extended imaging period at one angle with respect to the rotation axis might not be objectionable, where a large number (e.g. 60) of imaging periods at different angles are required to generate a tomographic image, the time required to complete an entire imaging session can be objectionable. In these cases, adding even a few seconds to each imaging period to achieve threshold photon levels can increase patient discomfort appreciably.
In addition, extended imaging sessions can lead to imaging errors. Ideally imaging should be performed while a patient remains completely still. If a patient moves during imaging, the resulting images can be blurred and their usefulness as diagnostic tools reduced. The likelihood of patient movement increases as imaging session duration increases.
One solution to increase imaging speed and hence throughput, is to increase transmission source activity thereby increasing the rate of intersecting photon absorption P.sub.AI. With a more active transmission source, threshold photon level T.sub.P can be achieved in less time thereby enabling a quicker scanning speed, shorter imaging periods T.sub.I and much shorter imaging sessions.
While a more active transmitter appears to be a viable solution for shortening imaging sessions without reducing image quality, unfortunately maximum transmitter activity is limited by imaging camera constraints. In particular, while camera processors are extremely fast, each process is only capable of processing a maximum number of detected photons per second MPPS where the MPPS can be expressed as: ##EQU2## where MCPS is a maximum number of calculations per second that the processor is capable of performing and C.sub.a is the number of calculations required to process each detected photon. If the number of photons generated by a transmission source causes the quantity of detected photons (i.e. P.sub.AI +P.sub.AN ; where P.sub.AN is the number of non-intersecting photons) to exceed the MPPS, the processor experiences "dead time" during which data related to some detected photons is effectively lost. Dead time results from the processor either failing to recognize essentially simultaneously detected photons and ignoring one of the photons or recognizing essentially simultaneously detected photons but processing light associated with both photons as a single photon thereby causing quantitative errors and image artifacts.
Because transmission source activity is limited, imaging speed is also limited. For example, at maximum allowable transmission source activity, the total number of detected photons should equal the MPPS such that: EQU MPPS=P.sub.AI +P.sub.AN Eq. 3
where P.sub.AI is the number of detected intersecting photons and P.sub.AN is a number of detected non-intersecting photons. Combining Equations 1 and 3, imaging period T.sub.I can be expressed as: ##EQU3##
Given a specifically sized torso segment to be imaged, the number of detected intersecting photons T.sub.P required to generate an image remains constant. In addition, given a specific processor, the MMPS remains unchanged. Moreover, given a specific segment slice to be imaged and a maximum allowable transmission source activity, the number of detected non-intersecting photons P.sub.AN is constant. Therefore, period T.sub.I cannot be reduced further using a conventional transmission imaging system.
As imaging speed is important, it would be advantageous to have an apparatus which can be used with a scanning line transmitter and a gamma camera to reduce the duration of an imaging session without decreasing imaging quality despite processor calculation limitations.